The present invention relates to a liquid crystal display panel and, more particularly, to a liquid crystal display panel which exhibits reliability by preventing defective sealing due to the shrinkage of an end-sealing material which seals a liquid crystal filling port.
Liquid crystal display devices are widely used for displaying various types of images, such as still images or kinetic images.
Liquid crystal display panels which constitute this kind of liquid crystal display device are generally divided into two types according to the structure of the electrodes which are formed over at least one of a pair of substrates which are opposed to each other across a predetermined gap into which liquid crystal compounds have been injected. One type is a simple matrix type. In the simple matrix type, two substrates each having a plurality of stripe-shaped transparent electrodes arranged over a main surface are opposed to each other so that the stripe-shaped electrodes of both substrates cross each other and the intersecting portions of the stripe-shaped electrodes constitute a two-dimensional matrix, each of the intersecting portion forming a pixel. The other group is called an active matrix type, and in this type electrodes formed over either one of two substrates are separated from one another for each pixel and an element having a switch function, such as a thin-film transistor, is provided for each pixel.
Active matrix liquid crystal display devices using active elements represented by thin-film transistors (TFTS) have been widely used as display terminals for OA equipment or the like because of their thin size and light weight, as well as their high image quality, which compares favorably with Braun tubes.
The display methods of liquid crystal display devices using thin-film transistors (TFTS) are generally classified into the following two methods. In one of the methods, a layer of liquid crystal compounds (hereinafter referred to simply as a liquid crystal layer or a liquid crystal) is interposed between two substrates (such as transparent glass substrates) with each substrate having a transparent electrode, and a voltage is applied across both transparent electrodes to generate a vertical field which will vary the direction of orientation of molecules in the liquid crystal layer, whereby fight which has passed through the transparent electrodes and enters the liquid crystal layer is modulated to display an image. This method is adopted by a considerably large number of current popular products.
In the other method, two electrodes are arranged on only one of the two substrates in a state such that they are spaced apart from each other, and a voltage is applied across the two electrodes so that a field nearly parallel, to the main surfaces of the substrates is generated in the liquid crystal layer to vary the direction of orientation of molecules in the liquid crystal layer, whereby light which has entered the liquid crystal layer through the gap between the two electrodes is modulated to display an image. Although there are few products which use this method, the method is a promising technique for active matrix liquid crystal display devices because of the ability to provide remarkably wide viewing angles.
A liquid crystal display device which adopts the former method is disclosed in, for example, Japanese Patent Laid-Open No. 309921/1988, and a liquid crystal display device which adopts the latter method and the features of such liquid crystal display device are disclosed in Japanese Patent Publication No. 505247/1993, Japanese Patent Publication No. 21907/1988 and the like.
All of the above-described active matrix types of liquid crystal display devices are provided with switching elements which respectively correspond to a plurality of pixel electrodes arrayed in a matrix form. In such an active matrix type of liquid crystal display device, since the liquid crystal in each pixel is theoretically driven at all times, good contrast is obtained compared to a simple matrix type of liquid crystal display device which is driven in a time-division manner. For this reason, the active matrix liquid crystal driving method is a technique which is particularly indispensable for color display.
A liquid crystal display device is prepared by bonding together two insulating substrates at least one of which is made of a transparent material such as glass, with a sealing agent along their respective peripheries, sealing a liquid crystal between the substrates to form a liquid crystal display panel, attaching constituent components such as a driving circuit, a polarizer and various optical sheets to the liquid crystal display panel, and incorporating the liquid crystal display panel together with the constituent components between a back fight assembly and a metal frame (a metallic shield case).
FIGS. 15A and 15B are explanatory views of a conventional structure of a sealing portion of such a liquid crystal display panel, and FIG. 16 is an enlarged view of a portion B of FIG. 15A.
In the liquid crystal display panel, two substrates SUB1 and SUB2 are opposed to each other across a predetermined gap and are bonded together by a sealing material SL inserted therebetween along the periphery of a display area. Incidentally, in the case of an active matrix type of color liquid crystal display panel, color filters for plural colors which are partitioned by a black matrix are formed over one of two substrates (an upper substrate) and switching elements for pixel selection are formed over the other substrate (a lower substrate).
The sealing material SL is stuck to the main surfaces of both substrates SUB1 and SUB2 along the peripheries thereof, and a portion of the sealing material SL is interrupted to form a filling port INJ for filling a liquid crystal. After the sealing material SL is cured, a liquid crystal is filled into the gap between the substrates SUB1 and SUB2 through the filling port INJ, and after that, the filling port INJ is sealed with the end-sealing material PLG.
The end-sealing material PLG enters into the gap between the substrates SUB1 and SUB2 through the filling port INJ and cross-bridges the sealing material SL, and sticks to each of the main surfaces of the substrates SUB1 and SUB2. If the end-sealing material PLG is cured in this applied state, the filling port INJ is hermetically closed.
A thermosetting or photosetting resin using an epoxy resin or the like is used as the end-sealing material PLG. In particular, a photosetting type of resin which can be cured by ultraviolet light or short-wavelength visible light is used in terms of quick curing and storage stability.
The photosetting resin (resin being set or cured by light-irradiation) is composed of only resin components such as a photo polymerization initiator, a cross bridging agent and a silane coupling agent.
The required characteristics of the end-sealing material are (1) adhesion to a liquid crystal sealing portion (compatibility to a liquid crystal; low shrinkage during curing) and (2) high purity and non-contamination.
These required characteristics will be further described. Regarding characteristics (1), the liquid crystal remains in the filling port INJ of the liquid crystal display panel and is difficult to completely remove by cleaning, so that the adhesion of the end-sealing material to the substrates lowers. If this adhesion is to be improved, it is necessary that the end-sealing material PLG be incapable of being repelled by the liquid crystal (affinity for liquid crystals), i.e., the end-sealing material PLG should have a certain degree of compatibility to liquid crystals.
In addition, if the shrinkage due to curing is excessively large, the end-sealing material peels off the substrates and its sealing characteristics lower.
Regarding characteristic (2), if a component of the end-sealing material PLG melts into the liquid crystal, a lowering of the resistance value of the liquid crystal is incurred and the retention efficiency of the liquid crystal decreases, so that display characteristics may become non-uniform in the screen. Because the end-sealing material contacts the liquid crystal in a non-cured state, it is particularly important to protect the liquid crystal from contamination.
In a sealing step for an end-sealing material using a photo-setting resin., in order to prevent constituent members (such as a liquid crystal, an alignment layer and a sealing material) of a liquid crystal display panel from being degraded by ultraviolet light, it is desirable that the amount of ultraviolet light required for curing be made as small as possible. If the liquid crystal, the alignment layer, the sealing material or the like is damaged by ultraviolet light, the display characteristics may become nonuniform in the screen.
As shown in FIG. 16, a resin RSN which constitutes the end-sealing material PLG is applied so that the resin RSN penetrates into the inside (in FIG. 16, the inside of the LCD) of the filling port INJ (between the two substrates SUB1 and SUB2) so as to bond together both substrates SUB1 and SUB2 and seal the filling port INJ, and, moreover, covers the filling port INJ at a side edge of the two substrates SUB1 and SUB2 which are bonded together (an outside edge of the substrates: in FIG. 16, the outside of the LCD).
In this manner, the filling port INJ is hermetically scaled and the liquid crystal is shut off from the outside atmosphere.